1. Field of the Invention
The present invention relates to the art of magnetic tunnel junction read heads, which sense magnetic fields in a magnetic recording medium. It finds particular application in conjunction with reading hard disk drives and will be described with particular reference thereto. However, it is to be appreciated that the invention will find application with other magnetic storage media. Further, it is to be appreciated that the invention will find application in other magnetic field detection devices as well as in other devices and environments.
2. Description of the Related Art
Magneto-resistive (MR) sensors based on anisotropic magneto-resistance (AMR) or a spin-valve (SV) effect are widely known and extensively used as read transducers to read magnetic recording media. Such MR sensors can probe the magnetic stray field coming out of transitions recorded on a recording medium by generating resistance changes in a reading portion formed of magnetic materials. AMR sensors have a low resistance change ratio or magneto-resistive ratio xcex94R/R, typically from 1 to 3%, whereas SV sensors have a xcex94R/R ranging from 2 to 7% for the same magnetic field excursion. SV heads showing such high sensitivity are able to achieve very high recording densities, that is, over several giga bits per square inch or Gbits/in2. Consequently, SV magnetic read are progressively supplanting AMR read heads.
In a basic SV sensor, two ferromagnetic layers are separated by a non-magnetic layer, an example of which is described in U.S. Pat. No. 5,159,513. An exchange or pinning layer of FeMn is further provided adjacent to one of the ferromagnetic layers. The exchange layer and the adjacent ferromagnetic layer are exchange-coupled so that the magnetization of the ferromagnetic layer is strongly pinned or fixed in one direction. The magnetization of the other ferromagnetic layer is free to rotate in response to a small external magnetic field. When the magnetizations of the ferromagnetic layers are changed from a parallel to an anti-parallel configuration, the sensor resistance increases yielding a relatively high MR ratio.
Recently, new MR sensors using tunneling magneto-resistance (TMR) have shown great promise for their application to ultra-high density recordings. These sensors, which are known as magnetic tunnel junction (MTJ) sensors or magneto-resistive tunnel junctions (MRTJ), came to the fore when large TMR was first observed at room temperature. See Moodera et al, xe2x80x9cLarge magneto resistance at room temperature in ferromagnetic thin film tunnel junctions,xe2x80x9d Phys. Rev. Lett. v. 74, pp. 3273-3276 (1995). Such MTJs have achieved an MR ratio of over 12%.
As the demand for ultra-high density recording grows, MTJ sensors seem likely to replace SV sensors in the near future. However, before that can happen, a new MTJ head structure is needed that can maximize the TMR properties.
Like SV sensors, MTJ sensors basically consist of two ferromagnetic layers separated by a non-magnetic layer. One of the magnetic layers has its magnetization fixed along one direction, i.e., the pinned layer, while the other layer, i.e., free or sensing layer, is free to rotate in an external magnetic field.
However, unlike SV sensors, the non-magnetic layer in MTJ sensors is a thin insulating barrier or tunnel barrier layer. Further, unlike SV sensors, MTJ sensors operate in CPP (Current Perpendicular to the Plane) geometry, which means its sensing current flows in a thickness direction of a laminate film or orthogonal to the surfaces of the ferromagnetic layers.
The sense current flowing through the tunnel barrier layer is strongly dependent upon a spin-polarization state of the two ferromagnetic layers. When the sense current experiences the first ferromagnetic layer, the electrons are spin polarized. If the magnetizations of the two ferromagnetic layers are anti-parallel to each other, the probability of the electrons tunneling through the tunnel barrier is lowered, so that a high junction resistance Rap is obtained. On the other hand, if the magnetizations of the two ferromagnetic layers are parallel to each other, the probability of the electrons tunneling is increased and a high tunnel current and low junction resistance Rp is obtained. In an intermediate state between the parallel and anti-parallel states, such as when the both ferromagnetic layers are perpendicular in magnetization to each other, a junction resistance Rm between Rap and Rp is obtained such that Rap less than Rm less than Rp. Using these symbols, the MR ratio may be defined as xcex94R/R=(Rapxe2x88x92Rp)/Rp.
The relative magnetic direction orientation or angle of the two magnetic layers is affected by an external magnetic field such as the transitions in a magnetic recording medium. This affects the MTJ resistance and thus the voltage of the sensing current or output voltage. By detecting the change in resistance and thus voltage based on the change in relative magnetization angle, changes in an external magnetic field are detected. In this manner, MTJ sensors are able to read magnetic recording media.
One problem with MTJ sensors is an enlarged read gap. U.S. Pat. No. 5,729,410 discloses an example wherein a MTJ sensor or element is applied to a magnetic head structure. The MTJ sensor is sandwiched between two parallel electrical leads or electrodes, which are in turn sandwiched between first and second insulating gap layers of alumina or the like to form a read gap. A pair of magnetic shield layers are further formed to sandwich therebetween the first and second insulating gap layers. In this example, the read gap is enlarged at a sensing or head end surface, i.e., an ABS (Air Bearing Surface), which confronts a magnetic recording medium. Thus, a MTJ head of this design is handicapped for application to high-density recording. Moreover, the biasing efficiency of this structure is poor due to the separation between the free layer and the permanent magnets. If the permanent magnets are formed in an overlapping manner on the TMR film, a strong decrease of the TMR ratio is yet expected due to a large difference of the junction resistance in the regions below and in between the permanent magnets.
U.S. Pat. Nos. 5,898,547, 5,898,548 and 5,901,018 disclose other examples wherein a MTJ sensor is applied to a magnetic head structure. In these publications, technical improvements are mainly proposed for adaptation to ultrahigh density recordings. However, the demand for development of MTJ heads for ultrahigh density recording has surpassed these improvements and proposals for more advanced TMR magnetic heads are demanded.
Another problem is a trade-off between high TMR ratio and MTJ resistance. The TMR ratio is proportional to the spin polarization of the two ferromagnetic layers. A TMR ratio as high as 40% was achieved by choosing a preferable composition for the two ferromagnetic layers. See Parkin et aL, xe2x80x9cExchange-biased magnetic tunnel junctions and application to nonvolatile magnetic random access memory, xe2x80x9d J App. Phys., v. 85, pp. 5828-5833 (Apr. 15, 1999). However, despite this large TMR ratio, the application of such MTJs in read heads was, up to now, prohibitory due to the large resistance of the junctions, resulting in a large shot noise Vrms and a poor signal to noise ratio S/N. Shot noise Vrms=(2xc2x7exc2x7Ixc2x7xcex94f)xc2xdxc3x97R, where: e=1.6xc3x9710xe2x88x9219 C; I=current; xcex94f=bandwidth; and R=junction resistance.
It is possible to reduce the MTJ""s resistance-area product Rxc2x7A or RA using a natural, in situ oxidation method. RA is a characteristic of an insulating barrier and contributes to junction resistance R through the equation R=Rxc2x7A/junction area. Using a 7 xc3x85 or less Al layer that is properly oxidized, an RA as low as 15 xcexa9xc2x7xcexcm2 has been achieved. This remarkably low value together with the high TMR ratio make MTJs very attractive for application as read heads for very high recording densities.
However, yet another problem in MTJs is that the thin insulating barrier is very sensitive to one of the manufacturing processes called lapping. Lapping involves the definition of an air bearing surface (ABS) on the MTJ head. Because the insulating barrier is so thin, lapping can create electrical shorts between the two adjacent magnetic layers, rendering the sensor useless. Actual designs thus include a front flux guide to protect the barrier during lapping. U.S. Pat. No. 5,898,547 proposed a design wherein the flux guide is made from the sensing layer. Obviously this design avoids any magnetic interruption between the flux guide and the sensing layer, which improves the magnetic efficiency of the flux guide. However, the large TMR ratio requires CoFe material within the sensing layer, and such large magnetization is inappropriate to get the highest efficiency of the flux guide. Thus, the design makes a compromise between high TMR ratio and high flux guide efficiency to get the largest output.
Therefore, a goal of the present invention is a read head design wherein the TMR ratio is be maximized by choosing MTJ materials with the largest spin-polarization and wherein the flux guide efficiency is optimized using hybrid low-magnetization materials to achieve a large signal output.
Another goal of the present invention is to provide a design wherein the junction area is enlarged to keep reasonable dimensions for high recording density, while maintaining the S/N at a high value.
Another object of the present invention is to provide a MTJ head with a high biasing efficiency and no reduction in TMR ratio to ensure a high and stable head output for adaptation to ultra high-density recording.
In accordance with one aspect of the present invention, an MTJ read head of the present invention comprises a magnetic tunnel junction read head having a sensing surface for sensing data magnetically recorded on a magnetic recording medium. The magnetic tunnel junction includes a tunnel barrier layer sandwiched between a ferromagnetic sensing layer and a ferromagnetic pinned layer. A ferromagnetic flux guide is magnetically coupled to the sensing layer. The flux guide extends to the sensing surface and has a magnetization lower than a magnetization of the sensing layer.
In a more limited aspect of the invention, the sensing layer is recessed from the sensing surface.
In another more limited aspect of the invention, the flux guide includes NiFeX where X is one or more of Ta, Nb, Cu, Cr, W, Al, Au, In, Ir, Mg, Rh, and Ru.
In yet another more limited aspect of the invention, the flux guide has a front portion and a back portion. The front portion has a width Ffw and extends generally along a length Fh from a front of the magnetic tunnel junction to the sensing surface. The back portion has a width Fbw and is adjacent the magnetic tunnel junction. Further, Ffw is less than a magnetic tunnel junction width Jw and Fbw is greater than Jw.
One advantage of the present invention is that the separate hybrid low magnetization flux guide greatly increases the output signal because the flux guide efficiency is improved. A high TMR ratio is maintained by the proper choice of materials involved in the sensing layer. Because the output signal is very high, the junction area can be significantly enlarged without degrading the S/N. This enables the design to function with very high recording densities beyond 100 Gbits/in2.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.